1. Field of the Invention
The present invention relates to refrigeration systems, and in particular to air chillers for cooling semiconductor devices under-test more efficiently.
2. Description of the Prior Art
One of the consequences of integrating millions of devices into a single microcomputer chip has been the heat generated by so many transistors. The heat represents wasted energy, and a modest amount of heat can shortened the service life of the appliance. Too much heat can destroy the electronics. Wasted energy and excess heating are being addressed on many fronts that include power management and more effective cooling systems. Technology limits are being pushed everywhere.
It used to be enough to heatsink a central processing unit (CPU) integrated circuit (IC) to the metal cabinet or other large metallic mass. Then finned aluminum heatsinks were necessary to be attached directly. Latter, more heat had to be disposed of by attaching small fans directly to the CPU heatsinks.
Still further advances in semiconductor device technology have made chilling them to their lower temperature limits during testing even more challenging. One commercial cooling system that has reached its limits recently is an air-chiller system that uses an exotic mixture of four refrigerants to chill a 10-CFM airflow down to −90° C. The cold airflow is directed onto the CPU heatsinks of high performance microprocessors. A typical test cooling system to do this draws 10-amps.
During testing and characterization, the newest generation of microprocessors needs higher 20-CFM airflows chilled to −90° C. What is needed today is a cooling system for these computers that can produce this doubled-volume of chilled-airflow for testing, but at only modest increases in power demand, e.g., 50% more. Increased chilled air volumes would also allow more devices to be tested in parallel.
A conventional air-chiller for this purpose is described by Dale Missimer in U.S. Pat. No. 3,768,273, issued Oct. 30, 1973, and titled SELF-BALANCING LOW TEMPERATURE REFRIGERATION SYSTEM, and incorporated herein by reference. It uses the familiar compressor, condenser, expansion valve, evaporator, and circulating refrigerants found in conventional air conditioning and refrigeration systems. Four refrigerants with different boiling points are mixed to get a multi-stage effect from the various liquid-vapor phase changes. Such patent describes using a mixture of 21.5 weight-percent (16.0 mol percent) trichlorofluoromethane (R-11), 21.5 weight-percent (18.2 mol percent) dichlorodifluoromethane (R-12), (23.8 wt percent) (23.1 mol percent) chlorotrifluoromethane (R-13), 30.2 weight-percent (35.0 mol percent) carbontetrafluoride (R-14), and, 3.0 weight-percent (7.7. mol percent) argon (R-740). Such fluorocarbons are, of course, no longer permitted in commercial use for refrigeration systems.
Prior art refrigeration systems like this can cool the refrigerants exiting the condenser to no less than the temperature of the ambient air being blown through the condenser. What is needed are better ways to cool down the compressed, liquefied refrigerants before they start their work in chilling the coolant air.